Magnetic resonance imaging (MRI) is primarily used in medical imaging to visualize anatomical structure of a patient's body. MRI technology can provide detailed images of the body in any plane. MRI has the ability to show soft tissue contrasts, which makes MRI scans especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging. MRI scans use a powerful magnetic field to align the magnetization of hydrogen atoms in the body. Radio waves are used to systematically alter the alignment of such magnetization, thereby causing the hydrogen atoms to produce a rotating magnetic field detectable by a scanning device of the MRI system. The resulting signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.
Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image or map of functional processes in a patient's body. A PET system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a metabolically active molecule. As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters, the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions, which are then detected when they reach a scintillator material in a scanning device of the PET system. Images of metabolic activity in space are then reconstructed by computer analysis.
The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other. Thus, it is possible to localize the source of the positron annihilation event along a straight line of coincidence (also referred to as a line of response (LOR)), and then an image reconstruction can be performed using coincidence statistics. For example, using statistics collected from tens-of-thousands of coincidence events, a set of simultaneous equations for a total activity of each parcel or bit of tissue (also called a voxel) along many LORs can be solved by a number of techniques, and thus a map of radioactivities as a function of location for parcels or bits of tissue can be constructed and plotted. The resulting map shows the tissues in which the molecular probe has become concentrated, and the resulting map can be interpreted by a physician and used for patient diagnosis and treatment.
PET scans are increasingly read alongside CT scans or MRI scans, in an attempt to produce a combination image by “co-registration” that gives the physician both anatomic and metabolic information about the patient's body. It is widely accepted that co-registration of anatomical information improves the diagnostic value of functional imaging, as can be seen in the success of hybrid scanners using PET and CT imaging. But the combination of PET and MRI may also offer advantages, such as higher soft tissue contrast in the MRI anatomical images, real simultaneous acquisition, and minimum radiation exposure to the patient. However, numerous obstacles have been present that have limited the ability to fully integrate PET and MRI systems into a combined scanner that produces accurate co-registration of the PET and MRI images.
Thus, there is a clear need for an improved method and apparatus for providing image alignment for combined positron emission tomography (PET) and magnetic resonance imaging (MRI) scanning.